How does a LASER work?

In my youth, I was an avid reader of Electronics Today International. Not only did it cover the more hefty electronic DIY projects but it also liked to look forward in terms of technology and covered items in far greater engineering depth that the likes of Tomorrow’s World which I also anjoyed.
One highlight of ETI was the Audiophile section which covered reviews and information on the latest hi-fi equipment. The author of this section had a thing for Felicity Kendal and usually printed a picture of her for no other reason that it was a very attractive picture.
I digress.
ETI once reported that it had been said of the LASER that “it was an invention looking for an application”. Up until recently, I had thought of this and what it meant. I now see it as an oddity that has many applications yet to be discovered but at the time, I took this to mean that it didn’t have many uses at all. My opinion changed when I saw an episode of the Avengers (From Venus with love, 1967, S5 E1). If the sight of Diana Rigg isn’t enough to whet your appetite, the plot involves astronomers being killed whilst looking at Venus. It turns out that they are being ‘zapped’ by a LASER and, if a popular programme can be considered a viable historical document, it goes on to state that LASERs are used for dental surgery, communications and eye surgery. This was certainly news to me that they were being used for these purposes in 1967 and there is something marvellous about seeing Mrs. Peel climb in to a Ford GT40 with a LASER mounted under the bonnet.
I digress again.

So what is a LASER and how, roughly, does it work?

LASER as I’m sure the reader knows is an acronym for Light Amplification by the Stimulated Emission of Radiation. It’s grounding, like so many modern inventions, is deep within quantum mechanics but can be explained without too much pain.

In a number of my articles, I’ve covered how electrons in an atom can be made to move to an outer shell by applying energy to an atom and how, when the electron returns to its original shell, it releases a photon. On the face of it, it is interesting but it does have many consequences and uses, some of which have been covered previously.
To recap:
Atoms have electrons arranged in shells that orbit the nucleus. The more electrons the atom has, the greater number of shells exist in which the electron orbits. Electrons can move to a higher or outer shell if exactly the right amount of energy is given to the atom. Too much or too little and nothing happens so the right level or quanta must be applied for the process to happen (this is the origin of the name quantum mechanics). Once up there, this energy level must be maintained else the electron will eventually drop down to its original shell and as it does so, will release a photon of exactly the same energy level as used to move it in the first place.

There are 3 ways of messing about with atoms with an eye to buggering around with the position of electrons.

Absorption is the process of moving an electron to an excited state by applying the correct energy level to an atom. Using the diagram below, the energy level would need to be E2 – E1.

Spontaneous emission is allowing the electron to drop down of its own free will and occurs at a random time after any absorption energy is removed. The photon will be of an energy level of E2 – E1

Stimulated emission is a combination of the 2. Should an electron be in an excited state and the atom is subjected to an energy level that is exactly that of the excited state minus its normal energy (its ground state) then the electron will move to its original state and release a photon – a kind of managed emission if you will. The incident photon energy will need to be E2 – E1. This method of emission was discovered by Einstein in 1916.

There are considerable difference between spontaneous emission and stimulated emission. Spontaneous emission is a random event and will emit a photon in some direction – any direction whereas stimulated emission will emit a photon in the same direction as the incoming or incident, at the same wavelength as the incident photon, with the same energy as the incident photon and at the same phase as the incident photon. All this means that the incident photon is effectively amplified by the emitted photon – we have light amplification!

So all we need is material with permanently excited electrons and a method of stimulating or ‘pumping’ this material in order to achieve stimulated emission. This is actually much easier than one would initially think and involves mirrors. In the diagram below, the amplifying medium – the material at the heart of the LASER – has on one side, a perfect mirror and on the other, a half mirror with the internals of the LASER being totally reflective. Pumping energy is therefore bouncing around inside the LASER whose length is an integral number of half wavelengths (in order to create a standing wave) and so keeps the atoms of the amplifying medium in an excited state. Photons may be absorbed and emitted a number of times before the half mirror provides an outlet. This, as well as the standing wave created within the LASER, means that the beam is a very refined sine wave which accounts for LASERs having a very confined focus and beam.

There are a huge number of equations and explanations that underpin the quantum mechanical operation of a LASER but the above is an overview that the author hopes is interesting enough to make the reader consider what is going on when a CD/DVD is being played or a cat being frustrated within an inch of its whiskers when chasing the elusive red dot.

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